1. Field of the Invention
The present invention relates to a variable valve timing control apparatus for an internal combustion engine, which apparatus includes an intermediate lock mechanism that locks a rotational phase of a camshaft relative to a crankshaft of the engine at an intermediate lock position. The rotational phase is referred to as a “variable cam timing (VCT) phase” in the present specification, and typically, the intermediate lock position is located between a full retard position and a full advance position of an adjustable range of the rotational phase.
2. Description of Related Art
In a conventional hydraulic variable valve timing device, as shown in JP-A-H9-324613 (corresponding to U.S. Pat. No. 5,738,056) and JP-A-2001-159330 (corresponding to U.S. Pat. No. 6,330,870), a lock position during engine stop is set at a generally middle phase within an adjustable range of a VCT phase such that the adjustable range of valve timing (VCT phase) is enlarged. In the above conventional art, the above intermediate lock position, at which the phase is locked during the engine stop, is set at a phase suitable for starting the engine. The engine is started while the VCT phase is at the intermediate lock position. Also, when oil pressure have been raised to a preferable pressure due to the increase of the engine rotational speed (oil pump rotational speed) after starting the engine, the lock is released such that valve timing (VCT phase) is feed-back controlled. In the above, an actual VCT phase is computed based on the pulse signals that are outputted synchronously with the engine rotation from rotation angle sensors (a cam angle sensor and a crank angle sensor). Thus, actuation oil pressure of the variable valve timing device is feed-back controlled such that the actual VCT phase, which has been released from the lock position, becomes a target VCT phase that is set in accordance with the engine operational state.
In the above, as described in JP3699654 (corresponding to US2002/0100442), in the computation of the actual VCT phase and the target VCT phase, the full retard position or the full advance position is set as a reference phase (0°CA) to compute the actual VCT phase and the target VCT phase.
As above, in the variable valve timing device having an intermediate lock mechanism, during the engine start operation of the engine, the engine is started while the VCT phase is locked at the intermediate lock position. Accordingly, after the engine start operation has been completed, it requires a certain amount of time before the engine begins to be operated under a specific operational state for learning the reference phase (the full retard position or the full advance position). Thus, until the reference phase has been learned after the engine start, the VCT phase has to be controlled with the unknown reference phase, and thereby it is impossible to accurately control the VCT phase disadvantageously.
Also, a recent on-board computer is typically provided with a back-up RAM that is capable of keeping data by using an on-board battery as the back-up power source even while the ignition switch is off (or while the engine is at rest or is deactivated). When the learning operation of the reference phase has been completed, data of the reference phase learning value is stored in the back-up RAM. Thus, the on-board computer will compute the actual VCT phase and the target VCT phase by using the reference phase learning value stored in the back-up RAM after the engine start in the next operation.
However, in an accident, where data stored in the back-up RAM is cleared due to the disconnection from the back-up power source caused by the erroneous detachment of the on-board battery while the ignition switch is off, data of the reference phase learning value is also cleared. As a result, during a period until the reference phase has been learned after the engine start, the VCT phase has to be controlled under a state, where the reference phase is unknown, and thereby it is impossible to accurately control the VCT phase disadvantageously.